Hitherto, austenitic heat resistant steel of carbide-nitride precipitation strengthening type, represented, for example, by JIS (Japanese Industrial Standard) SUH 35 alloy (SAE EV8) has been widely used as a material available for exhaust valves used in gasoline or diesel engines. Recently, since internal combustion engines, such as turbocharger engines, etc., have been improved so as to generate higher power, a nickel base alloy, such as so-called Inconel 751 (trademark of International Nickel Co., corresponding to SAE HEV3, hereinafter simply referred to as alloy 751), which has excellent strength at high temperatures, has been used in place of the SUH35 for the aforesaid utilities.
Alloy 751 has excellent hardness at high temperatures, i.e. not lower than a hardness of Hv150 at 850.degree. C., and a fatigue strength of 24 kg/mm.sup.2 at 850.degree. C. (at 10.sup.7 stress cycles). Thus, alloy 751 satisfies hardness at high temperatures and fatigue strength as an alloy for an exhaust valve to be used in aforesaid high power engines. On the other hand, alloy 751 has the drawbacks that it is expensive due to the high Ni content, i.e. 70%, and has poor sulfidation resistance at high temperatures as compared with above mentioned SUH35 alloy.
More recently, Incoloy 901 alloy, (trademark of International Nickel Co., hereinafter referred to as alloy 901) comprising 43Ni-13Cr-6Mo-3Ti-0.3Al has been proposed in order to surmount the foregoing defects by decreasing the Ni content to as low as 40 wt%, thereby lowering costs and improving sulfidation resistance at high temperatures.
Regardless of having a lower content of Ni, alloy 901 exhibits almost equal hardness at high temperatures to that of alloy 751 having not less than Hv150, but alloy 901 exhibits a lower fatigue strength than alloy 751, viz., 22 kg/mm.sup.2 at 10.sup.7 stress cycles at 850.degree. C. Furthermore, alloy 901 has a defect that the .gamma.' phase becomes unstable and .eta. phase (Ni.sub.3 Ti) is precipitated to embrittle the alloy, the creep strain rate is as large as 5.times.10.sup.-3 % h.sup.-1 at stress of 7.0 kg/mm.sup.2 thereby deforming a valve heat dishing into a cup form, and sulfidation resistance is not satisfactory.
As described above, prior known alloys do not fully satisfy desired properties with respect to hardness at high temperatures (e.g., 850.degree. C.), fatigue strength, sulfidation resistance, and creep strain rate.